Unit 2.5 Lecture 2

Question 1

What are lac and mal operons used for?

Answer 1

to metabolize two nutrients (lactose or maltose)

Question 2

When do bacterial cells need to synthesize proteins in the mal and lac operon that break down sugar?

Answer 2

if these sugars, lactose and maltose, are present

Question 3

What is lactose?

Answer 3

disaccharide of galactose and glucose

Question 4

What is maltose?

Answer 4

disaccharide of 2 glucose molecules

Question 5

A bacteria has a mutation removing the lacZ start codon. What will happen if this bacteria is transferred from media with only glucose to media with only lactose?

Answer 5

lac operon will be turned on and bacteria will stop growing

Question 6

What happens when there is low lactose?

Answer 6

no need for LacY or LacZ, LacI represses lac operon

Question 7

What happens when there is high lactose?

Answer 7

LacY or LacZ is important for lactose metabolism, LacI binds lactose, released from operator, operon transcribed

Question 8

Describe positive regulation. What happens if MalT is removed?

Answer 8

MalT protein activates transcription by helping RNA POL to bind

if MalT is removed, RNA POL cannot bind and no transcription occurs

Question 9

Describe negative regulation. What happens if LacI is removed?

Answer 9

LacI protein represses transcription by blocking the binding of RNA POL

if LacI is removed, RNA POL can bind and transcription can occur

Question 10

What is an operon?

Answer 10

cluster of genes encoded on a single mRNA but encoding several proteins

Question 11

What is polycistronic mRNA?

Answer 11

encodes more than one protein

Question 12

How is a prokaryotic promoter sequence different from an operator sequence?

Answer 12

-

Question 13

What does LacI protein do?

Answer 13

binds to operator but sometimes repressor releases from the operator even when lactose is absent

Question 14

What happens during the brief time when LacI is not bound to the DNA?

Answer 14

RNA POL can transcribe the operon and the level of expression is low (basal level)

Question 15

What is maltose?

Answer 15

a glucose disaccharide broken down by MalP and MalQ enzymes that produce energy

Question 16

When does the cell need to express the MalP and MalQ enzymes?

Answer 16

-

Question 17

What is the mal operon regulated by?

Answer 17

an activator called MalT that is constitutively expressed

Question 18

When is the mal operon off?

Answer 18

when maltose is low

Question 19

Where is the mal operator?

Answer 19

upstream of promoter

Question 20

RNA POL can’t bind the promoter easily on its own

Answer 20

-

Question 21

When is mal operon on?

Answer 21

when maltose is present

Question 22

Describe positive regulation of the mal operon.

Answer 22

1. MalT binds maltose
2. MalT changes shape
3. MalT binds the operator
4. RNA POL is recruited
5. transcription and translation of MalP and MalQ occurs
6. MalP and Mal Q break down maltose

Question 23

How does MalPQ operon recruit RNA POL?

Answer 23

the promoter is weak, so MalT is required to recruit RNA POL

Question 24

A mutation destroys the mal operator. Compared to a wild-type cell, what will the mutant phenotype be in the presence of maltose?

Answer 24

mutant will have less MalP production, compared to wild type

mutant knocks out MalPQ operon’s operator

Question 25

Sketch the curve of the ratio of active MalT to inactive MalT over time inside a bacterial cell before and after maltose is added to the cell. Assume that the amount of maltose added is limited and it is all consumed within the time shown. Explain what is happening to the MalT protein over time and how transcription of the mal operon changes.

Answer 25

• active form of MalT would increase very quickly once maltose is added and starts binding to the inactive form of MalT, converting to active form that turns on MalPQ operon
• once almost all MalT is activated, the graph would level off (maximum mal transcription)
• eventually the maltose concentration will decline due to the activity of MalP and MalQ, and the ratio of active to inactive MalT would decline until it reached the starting levels (decreasing mal transcription)

Question 26

What is an anabolic process?

Answer 26

pathways that construct molecules from smaller units

ie. arg operon: makes arginine amino acids

Question 27

What is a catabolic process?

Answer 27

breaks down molecules

ie. lac operon: breaks down lactose into glucose and galactose
ie. mal operon: breaks down maltose into two glucose molecules

Question 28

What happens if ArgR is an activator protein?

Answer 28

• it will help recruit RNA POL when arginine is absent

- when arginine binds to the ArgR protein, changing its shape, ArgR cannot bind the operator

Question 29

What happens if ArgR is an repressor protein?

Answer 29

• it will block RNA POL when arginine is present

- arginine binds to the ArgR protein, changing its shape, and allowing ArgR to bind the operator